Phase-Engineered WS Monolayer Quantum Dots by Rhenium Doping.

Transition metal dichalcogenides (TMDs) occur in the thermodynamically stable trigonal prismatic (2H) phase or the metastable octahedral (1T) phase. Phase engineering of TMDs has proven to be a powerful tool for applications in energy storage devices as well as in electrocatalysis. However, the mechanism of the phase transition in TMDs and the synthesis of phase-controlled TMDs remain challenging. Here we report the synthesis of Re-doped WS monolayer quantum dots (MQDs) using a simple colloidal chemical process. We find that the incorporation of a small amount of electron-rich Re atoms in WS changes the metal-metal distance in the 2H phase initially, which introduces strain in the structure (strained 2H (S2H) phase). Increasing the concentration of Re atoms sequentially transforms the S2H phase into the 1T and 1T' phases to release the strain. In addition, we performed controlled experiments by doping MoS with Re to distinguish between Re and Mo atoms in scanning transmission electron microscopy images and quantified the concentration range of Re atoms in each phase of MoS, indicating that phase engineering of WS or MoS is possible by doping with different amounts of Re atoms. We demonstrate that the 1T' WS MQDs with 49 at. % Re show superior catalytic performance (a low Tafel slope of 44 mV/dec, a low overpotential of 158 mV at a current density of 10 mA/cm, and long-term durability up to 5000 cycles) for the hydrogen evolution reaction. Our findings provide understanding and control of the phase transitions in TMDs, which will allow for the efficient manufacturing and translation of phase-engineered TMDs.